Verdantas (formerly Alden Research Laboratory) conducted computational fluid dynamics (CFD) and physical hydraulic modeling for the proposed Lake Ralph Hall Raw Water Pump Station. Designed for the Upper Trinity Regional Water District, the closed-conduit pump station will withdraw water from a new reservoir and convey it to an untreated water pipeline. The station was planned in phases, with four 18.33-million-gallon-per-day (MGD) pumps in Phase 1 and four 26.67-MGD pumps at ultimate buildout.  

Verdantas first used CFD modeling to evaluate three-dimensional flow patterns through the suction header and piping and identify hydraulic conditions that could affect pump performance. The analysis revealed strong spiral flow patterns as water branched from the header into individual pump take-offs, producing predicted swirl angles greater than 5 degrees. Verdantas then used CFD to evaluate multiple piping configurations and flow-straightening modifications, including cruciforms. The CFD results were used to identify the most promising cruciform location for subsequent testing in the physical model. 

Verdantas then constructed a 1:6-scale hydraulic model at its Holden, Massachusetts laboratory to evaluate flow patterns, vortex activity, swirl, velocity distribution, and hydraulic grade lines across a range of pump operating combinations. Testing was performed using acceptance criteria established by the Hydraulic Institute's ANSI/HI 9.8-2018 standard.  

Baseline physical testing confirmed unacceptable swirl conditions in both the Phase 1 and Phase 2 configurations. Using results from the CFD and physical models, Verdantas developed and tested modifications to improve approach flow to the pumps. Cruciform flow-straightening devices were incorporated into the final designs, reducing average swirl from as high as 12.6 degrees to 0.2 to 2.3 degrees in Phase 1 and from as high as 16.3 degrees to 0.5 to 2.0 degrees in Phase 2. The final configurations satisfied Hydraulic Institute acceptance criteria for all operating conditions tested, with no unacceptable subsurface vortices and acceptable velocity distributions.  

For both the baseline and modified configurations, the physical model results compared well with the CFD predictions, providing confidence in the use of CFD to evaluate subsequent design changes. After completing the physical model, the suction header diameter increased from 66 to 72 inches. Rather than reconstructing the physical model, Verdantas modified the CFD model to compare the revised geometry with the previously tested design. The analysis indicated similar swirl and velocity-distribution performance, providing additional information to support evaluation of the proposed design change.  

Project Highlights

• CFD analysis of three-dimensional flow patterns, swirl, and velocity distribution 
• Evaluation of multiple pump suction piping configurations and design modifications 
• 1:6-scale physical hydraulic modeling 
• Testing of Phase 1 flows up to 73.2 MGD and Phase 2 flows up to 106.8 MGD 
• Design-development testing of cruciform flow-straightening devices 
• Final configurations meeting Hydraulic Institute acceptance criteria for all operating conditions tested 
• CFD evaluation of a potential 72-inch suction header modification